Amplitude
The strength of a wireless signal. Amplitude is measured in dBm.
See: dB (Decibel).


Attenuation
The loss of signal strength that occurs as a wireless signal travels through the air and past (or through) obstructions located within the Fresnel Zone. See: Path loss, Fresnel Zone.

Bandwidth
Bandwidth and throughput are different. Bandwidth is the "raw" data rate of a wireless channel. For example, an 802.11b wireless access point has a "raw" bandwidth of 11 Mbps. The actual user data throughput may be only 5 to 6 Mbps however. The throughput of a system or a channel is always less than the "raw" data rate of the channel because of system "overhead".

Significance: It is important that you define and use "bandwidth" and "throughput" in a consistent way to maximize understanding and to meet or exceed the expectations of your wireless customers.

Beamwidth
A directional antenna focuses power into one main beam. This beam has a horizontal width (in degrees) when viewed from above and a vertical width (in degrees) when viewed from the side.

Significance: It's important to know 1) where a transmitted signal is going and 2) where received signals are coming from.

Corner Reflector
A directional antenna consisting of a dipole "driven element" and a corner-shaped reflecting element. Corner reflectors are quite directional and have a good front-to-back ratio.

dB (Decibel)
dB values are ratios of two powers. Standard dB reference terms include:
dBi - power gain of an antenna relative to an isotropic antenna
dBd - power gain of an antenna relative to a dipole antenna
dBm - power gain or loss relative to 1 milliwatt (1 mW)

Significance: Understanding dB measurement is essential for successful wireless work. Just as understanding how much gasoline (in gallons or litres) is in the tank of your car determines how far your car can go, understanding dB signal power determines how far your wireless signal can go.

Dipole Antenna
A dipole is a basic (half-wavelength long) "building block" antenna element. A dipole may be used by itself however it is usually used in combination with "director" elements and "reflector" elements to make a antenna system that has more gain and directivity than the dipole by itself.

Directional Antenna
A directional antenna focuses wireless energy in one primary compass direction.

Significance: Use directional antennas wherever possible.
This will minimize interference from other networks and maximize your communications distance. See: Omni-directional antenna.


DSSS (Direct Sequence Spread Spectrum)
A DSSS signal is a signal with a wide-band (wide frequency channel) signal with relatively low amplitude. It appears to be "noise" when received on a non-spread spectrum receiver.


EIRP (Equavalent Isotropic Radiated Power)
The EIRP is equal to the transmitter output power (in dBm), minus the power loss in the coaxial cable (in dB), plus the power gained from the directivity of the antenna (in dBi).

Significance: EIRP is normally higher than just the transmitter power because the gain of the antenna focuses the transmitter power and results in a higher ERP (or EIRP) than the transmitter power alone. FCC regulations allow a maximum of 4 Watts EIRP to be transmitted from an access point antenna.

ERP (Effective Radiated Power)
The ERP is equal to the transmitter output power (in dBm), minus the power loss in the coaxial cable (in dB), plus the power gained from the directivity of the antenna (in dBd).

Significance: ERP is normally higher than just the transmitter power because the gain of the antenna focuses the transmitter power and results in a higher ERP (or EIRP) than the transmitter power alone.

Fade Margin
The amount of "extra" signal (in dBm) above the minimum needed to barely establish a wireless link. The more extra signal, and the more reliable the link will be because the effects of fading will be overcome. Fade margin can be calculated during wireless system design and it can actually be measured during system testing. Fade margin is sometimes called System Operating Margin (SOM).

Significance: When you know the wireless fade margin, you can predict the wireless system reliability.


FHSS (Frequency Hopping Spread Spectrum)
An FHSS signal is a narrow bandwidth signal. FHSS signals regularly change frequency by hopping from frequency to frequency within the same frequency band.

Significance: You can deploy as many as 10 FHSS radios on the same rooftop as long as the radios use different hopping sequences.

F/B Ratio (Front-to-Back ratio)
The F/B ratio is the power ratio (in dB) between the power transmitted in (or received from) the favored direction (front) of an antenna compared to the power transmitted in (or received from) the back of the antenna. A vert good directional antenna will have a F/B ratio of 30 dB or more.

Frequency
Like the crests of ocean waves passing you, wireless energy passing a point change amplitude - high, low, high, etc. Imagine that these changes resemble a sine wave. The number of complete cycles of up-down-up change in one second is the signal's frequency.

One complete up-down-up cycle per second is called one hertz (Hz). One thousand Hertz (1000 cycles per second) is one Kilohertz (KHz). One million Hertz each second is one Megahertz (MHz). One billion (one thousand million) Hertz is one Gigahertz (GHz). The higher the frequency, the shorter the wavelength. See: Hz, KHz, MHz, GHz, Wavelength.

Fresnel Zone
Radio waves spread out after they leave an antenna. The
Fresnel (pronounced  'fra-nel' - the "s" is silent) zone is the area around the direct visual line-of-sight path. The radiated signal spreads out into this area. If less than 60% of the Fresnel Zone is not clear of obstructions, undersireable absorption and reflection will occur, causing attenuation (loss) of the wireless signal. See: Line-of-site.

Significance: The Fresnel zone must be clear of obstructions or the signal strength will be reduced because part of the signal will be lost (attenuated).


GHz (Gigahertz)
A frequency of one billion (one thousand million) hertz. See: Frequency.


Hz (Hertz)
One complete positive-negative-positive cycle of a wireless signal per second.
See: Frequency, KHz, MHz, GHz.e


Isotropic Antenna
A theoretical antenna that would radiate signals equally (like the sun) in all directions. An isotropic antenna is often used as a reference point to measure the power gain (the focusing power) of a real-world antenna, measured in dBi. See: dBi.


KHz (Kilohertz)
A frequency of one thousand hertz (1000 cycles per second). See: Frequency, Hz, MHz, GHz.


Line-of-sight
A wireless line-of-sight (LOS) path requires an unobstructed optical path plus a Fresnel Zone that is at least 60% clear of obstructions. See: Fresnel Zone.

Significance: You do NOT have a wireless LOS path unless you have BOTH a clear optical LOS and an unobstructed Fresnel Zone.


Link Budget
The sum of all the signal gains and losses over the entire wireless path, including the free-space path loss. See: Receiver Threshold, Free-Space Path Loss.

Significance: For reliable, fade-resistant wireless link performance, the link budget must be high enough to deliver a signal to the receiver that is higher than the receiver threshold.


LMDS
Local Multipoint Distribution Service. LMDS is a licensed wireless service that has the capability to provide broadband access.
It operates in the 29-32 GHz frequency range.


MHz (Megahertz)
A frequency of one million hertz. See: Frequency, Hz, kHz, GHz.


MMDS
Multichannel Multipoint Distribution Service. MMDS is a licensed wireless service that has the capability to provide broadband access. MMDS operates in several parts of the 2 GHz spectrum.


Multipath
The near-simultaneous reception of a direct signal and one or more reflected echoes of the signal. Multipath echos typically cause fading of the received signal.

mW
Milliwatt. One thousandth of a watt. 0 dBm is defined as 1 mW.


Omni-directional Antenna
An antenna that radiates equally well  in all directions. An omni-directional antenna is used when one site needs to connect to several surrounding sites.

Significance: Omni-directional antennae should only be used at the center of a hub-type network architecture. They reduce communications distance and increase the chance of interference. See: Directional Antenna.


Panel Antenna
A directional antenna made up of several driven elements
mounted in front of a flat reflecting element. A panel antenna
has a flat plastic or fiberglass cover which gives the antenna a panel-like appearance. Sector antennas (used at wireless access points) are one type of panel antenna.


Parabolic Antenna
A directional antenna made up of a driven element plus a parabolic-shaped reflector. The reflector may be either solid metal, metalic rods, or a metalic mesh.


Patch Antenna
A directional antenna that is a smaller version of a panel antenna. A patch antenna is most often used indoors. See: Panel Antenna.


Free-Space Path Loss
The attenuation of a wireless signal as it travels between the transmitter antenna and the receiver antenna. See: Attenuation, Fresnel Zone.


Polarization
The orientation relative to the earth of a wireless signal as it leaves a transmitting antenna. Polarization may be vertical, horizontal or circular.The polarization of the signal will change when the signal reflects off an object.


Receiver Threshold
The signal level needed for a receiver to begin to successfully detect and decode an incoming signal. See: Link Budget.


Sector
A wireless system that serves one specific coverage area from an access point (hub) site. Each sector has its own dedicated directional antenna system.


Signal-to-Noise Ratio
The ratio of the signal power to the noise (or the noise plus interference) power at the input of a receiver.

Significance: The signal power must be substantially higher than the signal-plus-interference level for the receiver to operate reliably.


Sensitivity
The ability of a wireless receiver to detect and successfully decode an incoming wireless signal.


Selectivity
The ability of a wireless receiver to discriminate between a wireless signal on the desired frequency and other wireless
signals on other frequencies.


SWR (Standing Wave Ratio)
SWR indicates the relative efficiency of an antenna system. The lower the SWR, the more power of the transmitter power that an antenna radiates and the better the wireless link performs.


Throughput
The amount of actual end-user data carried over a wired or wireless channel. Because of the "overhead" (or extra non-end-user) data required by a wireless link, the actual end-user data thrhoughput is always lower than the "raw" data rate of the channel.
See: Bandwidth.


Wavelength
Every wireless signal has both a specific wavelength and a specific frequency. The wavelength is the physical length of one cycle of the wireless signal. The higher the signal frequency, the shorter the wavelength. See: Frequency.

Yagi Antenna
A directional antenna made up of a "driven element" connected to the transmission line, a "reflector" (signal reflecting element), and one or more "directors" (signal directing elements).

(Updated 5/21/06)